1. Field of the Invention
This invention generally relates to gas-liquid contactors used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a wet flue gas desulfurization process and apparatus that uses an ammonia-containing scrubbing solution to remove sulfur dioxide and other acidic gases from flue gases, promotes the oxidation rate of the scrubbing solution to produce ammonium sulfate, and reduces the presence of free ammonia in the scrubbed flue gases.
2. Description of the Prior Art
Gas-liquid contactors are widely used to remove substances such as acidic constituents and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern is sulfur dioxide (SO.sub.2) produced by the combustion of fossil fuels and various industrial operations. Acidic gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes. The method by which acidic gases are removed with a gas-liquid contactor or other type of flue gas scrubber is known as wet flue gas desulfurization (FGD).
The cleansing action produced by gas-liquid contactors and absorbers is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs the acidic gases. A known configuration for a gas-liquid contactor 10 is shown in FIG. 1 as including an absorber tower 12 equipped with an inlet duct 14 through which combustion gases enter the tower 12. Shown above the inlet duct 14 are two banks of spray headers 16 which introduce a contact medium, e.g., an alkaline slurry or solution, into the tower. Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing liquids used in flue gas scrubbing operations. Additional banks of spray headers can be provided as may be required for a given application. A pump 20 recycles the contact medium from a tank 18 at the bottom of the tower 12 to the spray headers 16. Intimate contact between the contact medium and the flue gases rising through the tower 12 results in a cleansing action, after which the contact medium and the entrapped or reacted gases are collected in the tank 18 at the bottom of the tower 12. The cleansed gases continue to rise through the tower 12, then typically pass through a mist eliminator 22 and thereafter are either heated or passed directly to the atmosphere through an outlet duct 24.
While gas-liquid contactors and absorbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. In these processes, as the flue gases flow upward through the tower 12, acidic gases present in the gases are absorbed by an ammonium sulfate solution containing ammonia. Afterwards, the solution is accumulated in the tank 18, where the absorbed sulfur dioxide reacts with the ammonia to form ammonium sulfite (NH.sub.4).sub.2 SO.sub.3 and ammonium bisulfite (NH.sub.4 HSO.sub.3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate and ammonium bisulfate (NH.sub.4 HSO.sub.4), the latter of which reacts with ammonia to form additional ammonium sulfate. As shown in FIG. 1, oxygen and ammonia for these reactions are injected together into the tank 18 via a single conduit 26. A suitable source 28 for oxygen is air, and a suitable source 30 for ammonia is an anhydrous or aqueous ammonia solution. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction. A sufficient amount of ammonium sulfate is preferably removed from the ammonium sulfate solution prior to delivery to the tower 12 in order to maintain ammonium sulfate at a desired concentration in the solution.
In addition to being required to react with sulfur dioxide to produce ammonium sulfate, ammonia also serves to increase the efficiency of sulfur dioxide removal by reducing the acidity of the ammonium sulfate solution introduced into the tower 12. With the absorption of sulfur dioxide in the tower 12, the ammonium sulfate solution becomes more acidic and its ability to absorb sulfur dioxide is reduced. For example, without added ammonia the pH of the ammonium sulfate solution is generally in the range of about 4 and 5.5, but with added ammonia the solution generally has a pH of around 5 to 6, depending on control set points and operating conditions, including the SO.sub.2 concentration in the flue gas. However, oxidation of an ammonium sulfite solution is slower with higher pH levels.
Higher pH levels are also associated with the release of free ammonia from the solution, often termed "ammonia slip." In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gases reacts with uncaptured sulfur dioxide and trioxide to create an ammonium sulfate aerosol that is visible as a blue or white plume in the stack discharge, leading to secondary pollution problems. Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite that remain in the absence of sufficient oxygen. Therefore, high pH values and high levels of unoxidized ammonium sulfite promote ammonia slip.
In view of the above, an ongoing demand of desulfurization processes using ammonium sulfate scrubbing solutions is the ability to achieve efficient oxidation rates while reducing the release of free ammonia.